Intake vent for MUA placement

TakkoneMay 15, 2012

I have a 1,200 CFM canopy hood on the way. It will be installed on the inside of an exterior wall and vented straight up thru the roof, no turns. My contractor is suggesting I install an "intake vent with a damper on it directly behind the range, close to the floor, to provide makeup air. While I'm not 100% opposed to this, I don't think it is really needed. It is an older house with plenty of "holes" in it. The house also has a 1,500+ CFM whole house fan which we have used on High for years and have never had any carbon monoxide, backdraft, negative pressure, etc... type problems.

But, if I really should install the vent, is the proposed location a good one? Do I need to worry about cold/hot air seeping in during Winter/Summer months?

Is it just better to crack a window when needed?

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I don't believe you could pull an actual 1500 cfm from a house with closed windows and doors and not make the pressure negative (unless the house were Pentagon sized). (I believe it would be slightly negative even with windows open if they have screens.) Whether the developed differential pressure to the outside was sufficient to backdraft a particular appliance is another question. I would speculate that with the windows and doors closed, the negative pressure was sufficient to reduce the fan's actual cfm to some value that house leakage could support at that negative pressure.

Blade designs on many whole house fans do not have good performance at significant differential pressure, and if the fan empties into a well but typically ventilated attic, there will be further pressure loss on the output side.

With respect to dumping the air below the stove without any conditioning, the issue would be how cold can the cook stand the air at foot level in the winter. It won't be any worse than an equivalent window opening in the vicinity, but probably not a lot better.

Minimizing drafts, particularly when wind gusts hit the damper, could require multiple dampers or a powered damper or a pressure controlled damper. It might also help to box in the exterior port (much like a dryer vent cap but much larger) to resist direct wind impingement.


    Bookmark   May 15, 2012 at 6:59PM
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Thanks for the info, you are probably right about there being negative pressure when house fan is on.

Another question... Some companies make a electronically controlled open/close damper make up air vent. Is there a big advantage of going with something that auto-opens/closes vs a simple passive flap damper?

    Bookmark   May 16, 2012 at 3:10PM
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I really like the idea of putting the MUA vent under the range, does that placement actually work? Anyone have experience with that?

    Bookmark   May 16, 2012 at 5:45PM
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Takkone: An aided damper would open sooner than one that has to work against the spring that closes it. There might be slightly less pressure loss. I don't see a big difference for the case where the vent fan is pulling significant cfm. However, an aided damper would tend to not open due to pressure changes from wind when one wants it to stay closed.

SeaKoz: Someone on this forum did so, but you would have to search for a bit to find the thread on the subject. In my experience, using Google and setting the search location to "" generates more complete results. Some variations on "MUA", "under stove", and "under range" may be needed to get the desired link.


    Bookmark   May 17, 2012 at 10:01AM
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kaseki: Well said. I follow in your wake...

SeaKoz: Just as kaseki said, cold air is going to become an issue if you put the vent at toekick level. And even a damper with a motorized spring closure will still pass 3-5 CFM of air through them even if they are closed shut. The less intrusive your makeup air system is the happier you are going to be over the long term. Plus, putting the fresh air intake near the ground is going to draw in more dust, pollen, and anything you treat your yard with than fresh air. The higher you position the fresh air intake on the outside of your home, the cleaner the air will be.

    Bookmark   May 17, 2012 at 10:00PM
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